1. Field of the Invention
The present invention concerns a method and device for the detection of vocal signals which can be used, notably in alternate radio-electrical transmissions on board vehicles.
2. Description of the Prior Art
Most prior art detectors of vocal activity cannot work properly except for sufficiently high signal-to-noise ratios of the order of 20 dB at the minimum. This corresponds to working conditions in calm, office-type environments.
By contrast, on board a vehicle, the speech/noise discrimination has to take a far weaker signal-to-noise ratio, most usually lower than 10 dB, into account. Under certain conditions (high engine rate in a vehicle with average soundproofing, for example) the noise level may even exceed that of the signal.
Finally, the level and type of noise to be discriminated vary according to conditions inherent to the vehicle (the degree of soundproofing, for example) but also as a function of the route taken: a particularly unfavorable example is that of routes in cities where the noises to be taken into account are generally of a high level, are not stationary and are naturally highly varied.
An embodiment of a vocal activity detector designed to work in noisy environments is known from the patent application Ser. No. 79 74227 of 28th September, 1979, now U.S. Pat. No. 4,359,604 filed on behalf of the applicant. But this detector cannot be used to optimize speech/noise discrimination except for voiced sounds, and the decision is taken in comparing the vocal signal solely with a threshold voltage, this variable being automatically linked to the value of the peak amplitude of the vocal signal, without taking into account the real noise level. The result thereof is performance levels that do not suffice to enable proper operation in a highly disturbed environment where the speech signal is drowned in the noise.